The Jiangnan orogen, one of the largest gold-producing areas in China, has experienced multiple orogenic events with complex structural overprinting that is marked by multiple stages of magmatism, deformation, metamorphism, and orogenic gold mineralization. Different orogenic events have been recognized in the Neoproterozoic, mid-Paleozoic, Triassic, and Early Cretaceous, reflecting collisions and intracontinental orogenic episodes. The age of gold deposition in the Jiangnan orogen, however, has been poorly constrained owing to the absence of suitable dating minerals. Field studies in the orogen indicate the Tuanshanbei gold deposit includes two generations of auriferous quartz-ankerite-pyrite-arsenopyrite veins (Q2 and Q3), with the latter of the two notable for containing more abundant ankerite and base metal sulfides. The Q2 veins were formed throughout the near S-N–directed shortening associated with D1 deformation and along resulting subhorizontal to low-angle-dipping EW- to WNW-striking transpressive faults. The Q3 veins, containing about 70% of the total gold resource, were primarily localized in moderately to steeply dipping NW-striking tensional/tensional shear faults and moderately dipping NE- to NNE-striking transpressive faults that were products of NW-SE–directed shortening during D2 deformation. Both vein generations are temporally younger than the 437.2 ± 4.2 Ma Tuanshanbei granodiorite host, and both are crosscut by postgold ca. 225 Ma diabase dikes.
Hydrothermal monazite coexists with native gold and gold-bearing metal sulfides in the Q2 and Q3 veins. The Q2 monazite yielded a Tera-Wasserburg lower intercept age of 415.1 ± 2.1 Ma, consistent within error with an ankerite Sm-Nd isochron age of 410 ± 15 Ma and a laser ablation-inductively coupled plasma-mass spectrometry hydrothermal zircon 206Pb/238U age of 411.2 ± 4.0 Ma. The Q3 monazite yielded a Tera-Wasserburg lower intercept age of 234.3 ± 1.1 Ma. These new ages suggest that the Early Devonian gold event was overprinted by hydrothermal activity along the same structural system almost 200 m.y. later such that the gold resource must be a product of two temporally distinct events. Geologic and structural evidence, coupled with existing published geochemical data, suggests both ore-forming events were related to crustal metamorphism typical of most orogenic gold deposits. Fluids would have been derived from Neoproterozoic metasedimentary basement rocks, most likely from metamorphic devolatilization of the Neoproterozoic Cangxiyan Group greenschist-amphibolite facies metasediments. There is no evidence suggesting any type of magmatic contribution to the ore-forming process at either time. The data are best interpreted to suggest that various parts of the basement were metamorphosed near the greenschist-amphibolite boundary at different times, but during both times, the gold-bearing metamorphic fluids eventually migrated into the same structural conduits.
In contrast to many orogenic gold deposits that formed in convergent plate margins (accretionary orogens: Groves et al., 1998; Goldfarb and Groves, 2015), gold deposits in the Jiangnan orogen formed in an intracontinental orogenic setting (Wang et al., 2013; Ni et al., 2015) defined by a suture between two Precambrian blocks that was reactivated multiple times during subsequent continental margin far-field stresses. The Jiangnan orogen was created by the Neoproterozoic collision between the Yangtze and Cathaysia blocks (Fig. 1A; Cawood et al., 2013; Xu et al., 2017a, b; Yao et al., 2019; Shu et al., 2021). It was characterized by multistage magmatism, deformation, metamorphism, and gold events (Figs. 1, 2; Pirajno and Bagas, 2002; Wang et al., 2013; Zhang, L., et al., 2019) and hosts more than 250 gold deposits and occurrences with a combined resource of >970 tons (t) gold (e.g., Xu et al., 2017a; Zhang, L., et al., 2019). The timing and genesis of the gold deposition is highly debated, in large part because of the various magmatic and metamorphic events in the Jiangnan orogen (e.g., Ni et al., 2015; Xu et al., 2017a; Zhang, L., et al., 2018, 2019; Wang et al., 2020a, b). Resolving the mineralization ages and their relationship to the geodynamics of the region is a fundamental step for understanding the ore genesis (Rasmussen et al., 2006; Gao et al., 2021). Isotopic dating of many of the deposits, using methods that include Pb-Pb, K-Ar, 40Ar/39Ar, Rb-Sr, Re-Os, (U-Th)/He, and electron spin resonance (ESR), has led to published data regarding temporal relationships that are inconsistent (Zhang, L., et al., 2019). Accurate dating of gold deposition in the Jiangnan orogen is still a challenge because of the prolonged orogenic episodes in the Neoproterozoic, middle Paleozoic, and Triassic, consequential overprinting hydrothermal events, and late Mesozoic extension, all coupled with the lack of suitable minerals for radiometric dating that are clearly related to gold mineralization.
The nearby presence of deformation zones with overprinting events and previously dated intrusive rocks make the Tuanshanbei deposit an ideal target to investigate the genetic relationship between gold mineralization, deformation, and magmatism (Figs. 1–3). The exploration at Tuanshanbei started in the 1980s, and reserves of 12.03 t gold were initially reported during the 1990s to 2000s (214 Brigade of Hunan Non-Ferrous Metals Geology Investigation Bureau, unpub. report, 2009). Since 2009, average annual gold production from Tuanshanbei has been 1.5 t, and approximately 80 t gold is now suggested as the remaining resource in the deposit (Hunan Institute of Geological Survey, unpub. report, 2018). Gold orebodies are EW-WNW–, NW-, and NE-NNE–striking, and several ore-bearing veins are hosted in granodiorite and are crosscut by diabase dikes (Figs. 4, 5). The crosscutting relationship have broadly constrained the timing of mineralization (Fig. 5); however, the absolute age(s) for the gold mineralization remains uncertain. To address this, we conducted detailed mineralogical studies on the Tuanshanbei gold deposit and identified trace amounts of hydrothermal monazite, zircon, and ankerite either intergrown with or enclosed in gold and related sulfide minerals.
Monazite is an ideal mineral for isotopic dating because of its high Th and U contents, negligible common Pb, high closure temperature for U-Pb diffusion (>750°C), and high resistance to metamorphic resetting (e.g., Rasmussen et al., 2006; Williams et al., 2007; Cherniak, 2010; Grand’Homme et al., 2017). Monazite is commonly formed during hydrothermal activity by dissolution and reprecipitation reactions at temperatures of less than 400°C (Townsend et al., 2000; Rasmussen et al., 2007; Deng et al., 2020). Radiometric dating of monazite to determine age of gold deposition has been successfully conducted at many gold deposits during the past decade (e.g., Sarma et al., 2011; Fielding et al., 2017; Deng et al., 2020; Qiu et al., 2020; Liu et al., 2021). Hydrothermal zircons have also been identified in some auriferous veins and have been used for high-precision dating of gold mineralization (e.g., Claoué-Long et al., 1990; Pelleter et al., 2007; Bao et al., 2014). Finally, ankerite is a common gangue mineral in the gold deposits of the Jiangnan orogen, and application of Sm-Nd dating methodology is an additional candidate for constraining the age of gold mineralization (e.g., Su et al., 2009; Liu et al., 2014; Maas et al., 2020).
Our petrographic work has revealed that gold was deposited synchronously with coexisting hydrothermal monazite, zircon, and ankerite in the Tuanshanbei gold deposit. These minerals are key for probing the potential relationships between the gold mineralization, deformation, and magmatism and for further understanding the hypothesized multistage mineralization associated with orogenic events in the Jiangnan orogen. This paper provides a structural geology framework to evaluate the first in situ laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb age data for hydrothermal zircon and monazite, as well as ankerite Sm-Nd isochron ages for the deposit. Our data reveal the absolute timing of gold mineralization and define two periods for the gold mineralization in the central Jiangnan orogen that locally cause overlap of features at the Tuanshanbei deposit, further constraining the genetic links of gold deposition with the regional orogenic events.
Geology of the Jiangnan Orogen
The NE-ENE–trending Jiangnan orogen in south China was formed by the Neoproterozoic assembly of the Yangtze and Cathaysia blocks, following the closure of the Paleo-South China Ocean and collision of the blocks during the period from 980 to 820 Ma (Fig. 1A; Yao et al., 2019; Shu et al., 2021). The orogen is bounded to the south by the Shaoxing-Jiangshan-Pingxiang fault and to the north by Phanerozoic sedimentary rocks (Zhang, L., et al., 2019). Much of the orogen is composed of Neoproterozoic greenschist facies schist, phyllite, and slate, as well as granitoids and mafic igneous rocks (Li et al., 2016). The belt has endured a complex tectonic evolution since the two microcontinents were combined into the South China block. This included (1) late Neoproterozoic collision and subsequent breakup documented by collisional S-type granitoids (845–815 Ma), a regional-scale angular unconformity (ca. 820–795 Ma), early top-to-the-southeast thrusting and sinistral strike-slip shearing, rift-related bimodal magmatism (810–760 Ma), and regional medium-grade metamorphism (likely at ca. 820–815 Ma; Shu et al., 1995; Shu and Charvet, 1996; Yao et al., 2019); (2) early Paleozoic intracontinental orogeny (ca. 464–380 Ma) marked by an angular unconformity between the pre-Silurian and Middle-Late Devonian sequences (Hunan Bureau of Geology and Mineral Resources, 1988), near east-west fold-andthrust development with subsequent strike-slip deformation, and S-type granitoid magmatism and migmatization (Wang et al., 2013; Shu et al., 2021); (3) Triassic intracontinental orogeny represented by the reworking of Devonian and older structures, as well as initially formed large-scale NNE- to NE-striking (notably in the western of the Jiangnan orogenic belt) fold-and-thrust and ductile strike-slip shearing features (ca. 240–200 Ma; peak age at ca. 230 Ma), which were coeval with the peak of regional granitic magmatism and an angular unconformity between the pre-Triassic and Upper Triassic sequences (Wang et al., 2005, 2013; Shu et al., 2015); and (4) late Mesozoic extensional tectonics after the transition from Tethyan to paleo-Pacific tectonic domains, which led to a Basin-and-Range-like topography, with multisourced igneous rocks and coeval metamorphic core complexes and normal fault-bounded granitic domes (Li et al., 2016; Ji et al., 2018; Zhang, K., et al., 2019).
In the west to central part of the Jiangnan orogenic belt, metamorphism of the Phanerozoic rock sequence is weak, whereas the Precambrian basement rocks (i.e., the Neoproterozoic Lengjiaxi and Banxi Group and its equivalents) are primarily metamorphosed to lower greenschist facies (Hunan Bureau of Geology and Mineral Resources, 1988). However, in addition and more locally, the Cangxiyan Group basement rocks were metamorphosed to greenschist to amphibolite facies in scattered exposures in northeastern Hunan province (Tang et al., 1999; Wu et al., 2005; Gao et al., 2011). In contrast, Phanerozoic rocks are metamorphosed from greenschist to granulite facies within the eastern part of the Jiangnan belt and in the adjacent Yunkai-Baiyunshan-Wuyi domains in the Cathaysia block (Fig. 1A; Bureau of Geology and Mineral Resources of Fujian Province, 1985; Bureau of Geology and Mineral Resources of Guangdong Province, 1988; Bureau of Geology and Mineral Resources of Zhejiang Province, 1989). The Phanerozoic metamorphism generally shows a broad trend of decreasing metamorphic grades from the Cathaysia block to the northwest and thus into the eastern Yangtze block. Zircon and monazite ages of the greenschist to granulite facies metamorphic rocks can be roughly grouped into periods of 460 to 390 and 252 to 218 Ma, thus corresponding to two periods of intracontinental orogeny (Wang et al., 2013).
Regional Geology of Northeastern Hunan Province
Gold-rich northeastern Hunan Province is located in the central part of the Jiangnan orogen, which is characterized by a Basin-and-Range-like topography and bounded by the Xinning-Huitang-Gongtian normal fault to the northwest and the Liling-Hengdong thrust and sinistral strike-slip fault to the southeast (Fig. 1B; Bureau of Geology and Mineral Resources of Jiangxi Province, 1984; Rao et al., 1993; Li et al., 2016; Zhang, L., et al., 2019). The widely exposed strata in the area are dominated by the Neoproterozoic Lengjiaxi Group metasedimentary sequences, i.e., greenschist facies clastic metasandstone, metasiltstone, phyllite, and slate, with various metatuffaceous layers (Shu et al., 2021), and Sinian-Tertiary conglomerate, sandstone, siltstone, shale, and carbonate (Figs. 1–3; Gao et al., 2011). The poorly exposed Cangxiyan Group metamorphic rocks unconformably underlie the Lengjiaxi Group strata and were deposited at 855 ± 5 Ma (SHRIMP zircon U-Pb age; Gao et al., 2011; Fig. 2); local outcrops occur along the east side of the Changsha-Pingjiang fault. The Cangxiyan Group rocks comprise a series of greenschist to amphibolite facies schists with volcanic-sedimentary protoliths and interlayers of tremolite rock, gneiss, and amphibolite (Tang et al., 1999; Che et al., 2005; Wu et al., 2005; Gao et al., 2011).
The dominant structural framework is defined by NE-ENE–trending regional folds and faults in the Neoproterozoic metasedimentary rocks and Sinian-Phanerozoic sedimentary sequences and folds with WNW– to E-W–trending axial surfaces and related faults in the Neoproterozoic metasediments and pre-Devonian sedimentary sequences (Fig. 1B). The NE-ENE–trending regional structures are accompanied by less extensive NW-striking faults formed often as cross faults that cut the earlier WNW– to E-W–trending folds, fold-related faults, and brittle-ductile shear zones (Hunan Bureau of Geology and Mineral Resources, 1988; Zhang, L., et al., 2019).
Intrusions in the region (Fig. 1B) are reflective of multiple periods of magmatism. Neoproterozoic intrusions in the central part of the orogen (Figs. 1B, 2) include the Sandun, Meixian, and Daweishan, Getengling, and Zhangfang granites (Li et al., 2001, 2008; Zhong et al., 2005; Hunan Bureau of Geology and Mineral Resources, 2017) and metabasites of broadly similar age such as the Cangxi amphibolites and Nanqiao metabasalts (Zhang et al., 2013; Hunan Bureau of Geology and Mineral Resources, 2017). Paleozoic intrusions include the large Hongxiaqiao and Banshanpu granodiorites (434–432 Ma; Guan et al., 2013), which are located a few tens of kilometers southwest of the studied Tuanshanbei deposit (Fig. 3). Mesozoic intrusions are more distal to the studied deposit (Fig. 1B) and include the Wangxian granodiorite (224.7 ± 4.4 Ma; Yang et al., 2018), Yajiangqiao monzogranite (224–212 Ma; Li et al., 2019; Yu et al., 2019), Mufushan (158–125 Ma; Ji et al., 2018; Xiong et al., 2020), Lianyunshan (150–140 Ma; Wang et al., 2016), Wangxiang (151–128 Ma; Hunan Bureau of Geology and Mineral Resources, 1988; Jia et al., 2003), and Jingjin granites (166–144 Ma; Hunan Bureau of Geology and Mineral Resources, 1988).
Northeastern Hunan Province, with an estimated resource of >315 t gold, is the most important gold-bearing area in the Jiangnan orogen (Lu et al., 2020), hosting orogenic gold deposits that include Huangjindong, Wangu, Zhengchong, Yanlinsi, and Tuanshanbei (App. Table A1; Fig. 1B; Xu et al., 2017a; Zhang, L., et al., 2018, 2019). The deposits are mainly hosted in the Neoproterozoic metasedimentary rock sequences where they are adjacent to granitoids (Wang et al., 2020a). Individual gold fields (e.g., Huangjindong and Wangu gold fields; Fig. 1B) are aligned along both sides of the crustal-scale NE-striking Changsha-Pingjiang fault, which defines a first-order structural control on the distribution of the gold orebodies (Zhang et al., 2018; Zhou et al., 2021). This long-lived fault zone first developed during the Neoproterozoic collision between the Yangtze and Cathaysia blocks during regional northwest-southeast shortening, and it was reactivated by Devonian and Triassic sinistral strike-slip movement. Jurassic early sinistral and late dextral movement and associated Cretaceous normal movement also were recorded (Li et al., 2013; Zhou et al., 2021). In addition to the numerous gold deposits in the region, several magmatic deposits are also located in northeastern Hunan (Fig. 1B), including the Renli-Chuanziyuan Li-Nb-Ta and Baishawo Be-Li-Nb-Ta pegmatite-related ores (Xiong et al., 2020; Li et al., 2021; Wen et al., 2021), Qibaoshan Cu-Pb-Zn deposit (Yuan et al., 2018), and Jinchong Co-Cu deposit (Wang et al., 2017).
The Liuyang-Liling gold field hosting the studied Tuanshanbei deposit is located in the footwall of the NW-dipping Changsha-Pingjiang fault zone and is about 70 km away from the main exposure of the fault. It is also located in the hanging wall of the NW-dipping Liling-Hengdong fault and centered about 10 km to the west of the fault exposure (Fig. 1B). The crustal-scale Liling-Hengdong fault is exposed for more than 150 km along a northeast-southwest strike marked by the distribution of mafic-ultrabasic rocks and has an evolutionary history similar to that of the Changsha-Pingjiang fault zone (Rao et al., 1993). Each of these deformation events, particularly the Devonian and Triassic reactivations, potentially may have influenced transport of fluids and metals (Rao et al., 1993; Zhang et al., 2018; Zhou et al., 2021). Locally, at the deposit scale in the Liuyang-Liling gold field, individual orebodies are distributed along EW- to WNW- and NE- to NNE-trending transpressive faults or their associated bedding-parallel faults and cleavage zones, which are parallel to the axial plane of overturned folds, and NW-trending tensional/tensional shear faults that are perpendicular to the fold axes of NNE- to NE-trending folds (Table 1; Figs. 1B, 3; Zhang, L., et al., 2019; Zhou et al., 2021; Wang et al., 2020a).
Tuanshanbei Deposit Geology
The Tuanshanbei gold mineralization is predominantly hosted within the Neoproterozoic Lengjiaxi metasedimentary rock sequence and, to a lesser degree, in the ca. 437 Ma Tuanshanbei intrusion (Figs. 4–6). Where the medium- to fine-grained and foliated granodiorite host rock is the ore host, it is composed of quartz, biotite, K-feldspar, plagioclase, and hornblende, which are locally hydrothermally altered to an assemblage including sericite, chlorite, pyrite, and arsenopyrite (Fig. 6D). Several subhorizontal to low-angle-dipping EW- to WNW-striking gold-bearing veins are cut by moderately to steeply dipping NW-striking and moderately dipping NE-striking gold-bearing veins (Figs. 4, 5). Diabase dikes (225.2 ± 1.9 Ma; Zhou et al., 2022) cut the granodiorite and both generations of the auriferous veins (Fig. 5E, F). The dikes are dark black to green in color and have an ophitic texture, comprising pyroxene (1–2 mm) and plagioclase (1–5 mm). Quenching boundaries are developed along the contact between the granodiorite and the diabase dikes (Fig. 5E, F).
Quartz-ankerite veins are widely distributed ore hosts in the Tuanshanbei deposit. Based on their morphology and textures, veins can be divided into three types (Figs. 5–7): early barren quartz veins (Q1), subhorizontal to low-angle EW- to WNW-striking auriferous quartz veins with minor ankerite (Q2), and moderately to steeply dipping NW-striking and moderately dipping NE-striking auriferous quartz-ankerite veins (Q3), which typically cut the Q2 veins but locally are parallel to the margins of the Q2 veins. Whereas the Q2 and Q3 veins are the economic targets being mined, no estimate has been released by the Jinsha mining company regarding the percentage of ore in each generation when calculating the present gold resource. We estimate roughly 30% of the gold resource is hosted in the Q2 veins and 70% in the Q3 veins based on the internal company production reports, discussion with mine personnel, and our own observations.
The Q1 barren quartz veins, with lengths of 1 to 2 m, predominantly dip to the south or north in the granodiorite. They are lenticular and comprise quartz with minor pyrite and arsenopyrite (Fig. 5A-D). The Q1 veins contain abundant irregular breccia fragments and are cut by later gold-bearing veins (Fig. 6B). Their irregular occurrence solely in the intrusive rock and their barren nature suggest the Q1 veins to be magmatic-hydrothermal veins filling fractures in the granodiorite and with no relationship to the younger gold-bearing vein sets.
The Q2 auriferous and sulfide-bearing veins occur in both the granodiorite and surrounding Lengjiaxi Group slate with subhorizontal to low-angle dips and east-west to west-north-west strikes. They typically fill preexisting bedding faults or low-angle fault zones and have lengths of 200 to 300 m, widths of 1 to 3 m, and average gold grades of 6.3 g/t. The Q2 veins cut the lenticular Q1 veins (Figs. 5A-D, 6B) and are cut by Q3 veins and diabase dikes (Fig. 5A-D). Minerals in gold-bearing Q2 veins include pyrite, arsenopyrite, quartz, and minor gold, stibnite, pyrrhotite, ankerite, calcite, monazite, zircon, and rutile (Figs. 6A-G, 7A-C). Pyrite and arsenopyrite are euhedral to subhedral and fine grained (10–100 μm). They commonly occur in aggregates and also coexist with ankerite and quartz (Figs. 6E-G, 7A-C). The sulfide aggregates may occur as black and gray lines in the quartz veins. Quartz in the Q2 veins is pale gray and white in color and coarse grained (>5 mm). Minor ankerite (Fig. 6E), coexisting with quartz and sulfides, is irregular and medium to coarse grained (1–5 mm).
The Q3 auriferous and sulfide-rich veins commonly have higher gold grades (avg 11.1 g/t) than Q2 veins. The Q3 veins are predominantly northwest striking (290°– 340°) and northeast striking and cut the Q2 veins (Fig. 5); some Q3 veins and veinlets fill fractures in Q2 veins (Fig. 6C). The Q3 veins consist of pyrite, arsenopyrite, chalcopyrite, sphalerite, galena, ankerite, and minor gold, quartz, calcite, monazite, zircon, rutile (Figs. 6G-I, 7D-I), and Bi-, Sb-, Au-, Te-, and Ni-bearing minerals (e.g., heteromorphite [Pb7Sb8S19], bismuthinite, cosalite [Pb2Bi2S5], famatinite [Cu3SbS4], jonassonite [AuBi5S4], aurostibite [AuSb2], hedleyite [Bi7Te3], and ullmannite [NiSbS]). The abundance of ankerite, base metal sulfides, and sulfosalts provides a strong contrast with observations of the Q2 veins. Ankerite (0.5–3 cm) is the dominant gangue mineral in the Q3 veins (Fig. 6G-I). Pyrite and arsenopyrite are euhedral and fine grained. Chalcopyrite is irregular and medium to coarse grained, coexisting with famatinite, cosalite, sphalerite, heteromorphite, bismuthinite, antimonian bismuthinite, antimonian bismuth, jonassonite, ullmannite, aurostibite, and gold. Cosalite is euhedral and needle shaped with medium to coarse grains (Fig. 7F) and predominantly coexists with chalcopyrite. Famatinite is euhedral to subhedral and fine grained, coexisting with sphalerite (Fig. 7G). Heteromorphite, bismuthinite, antimonian bismuthinite, antimonian bismuth, and jonassonite commonly coexist isomorphically in subhedral and fine-grained textures (Fig. 7H, I). Gold is generally fine to medium grained (5–150 μm) with irregular shape and coexists with chalcopyrite, aurostibite, ullmannite, quartz, monazite, and ankerite (Figs. 7D, E, 8D-F). Ullmannite is anhedral and fine grained, coexisting with gold and chalcopyrite in the ankerite-quartz veins (Fig. 7D). Monazite is euhedral to subhedral and fine grained, filling fractures or forming a cojunction texture with pyrite (Fig. 8D, E).
Ore-related hydrothermal alteration phases comprise quartz, carbonate, sericite, and chlorite, with the ore-hosting granodiorite (2–10 cm in halo widths) showing a stronger degree of alteration than the slate (0.5–2 cm in halo widths). The Q3 veins contain more ankerite, sulfides, sericite, and chlorite and conversely less abundant quartz than the Q2 veins. Sericite and chlorite are extensively developed alteration phases in the granodiorite surrounding gold-bearing veins. Disseminated sulfides, dominated by pyrite and arsenopyrite, with minor chalcopyrite, sphalerite, and galena, are also locally observed within the altered host granodiorite (Fig. 6D). In contrast, wall-rock chloritization and sulfidation are rarely developed in the slate, but the sericitization is more extensive. In addition, silicification is extensively observed surrounding the Q2 and Q3 veins within the host granodiorite, whereas it is much less pervasive in the slate (Fig. 6). Carbonate, consisting of ankerite and minor calcite, is also a common alteration mineral in and surrounding auriferous quartz veins in both the host granodiorite and the slate (Fig. 6E-I). Sericitization is observed in microcracks in the quartz veins (Fig. 6). Chlorite grains are clustered along the margins of the sulfide-bearing quartz veins (Fig. 6A-C).
Structural Analysis of the Tuanshanbei Deposit Area
Field studies in the Liuyang-Liling gold field, which include mapping at the Tuanshanbei and adjacent gold deposits (Figs. 1–3; App. Table A1), show evidence of two major and very distinct phases of ore-controlling deformation (Figs. 3, 9; Table 1). We conducted kinematic analyses and petrographic observations to define deformation patterns and tectonic chronologies in the gold field.
D1: Near S-N–directed shortening
Our field observations and mapping have revealed that nearly EW- to WNW-trending strain zones are widely developed in the Neoproterozoic Lengjiaxi Group strata in the Liuyang-Liling gold field (Figs. 3, 9). These zones are as much as 10 km in length and tens to hundreds of meters in width. Shortening structures include folds of different scales, moderately to steeply dipping S1 cleavages, and thrust faults identified in the Neoproterozoic Lengjiaxi Group slates and metasandstones (graywackes) (Fig. 9). One main thrust fault studied in detail within the Lengjiaxi Group slates (Fig. 9A) dips at 55° to the southwest. Strata in the footwall are gently dipping at 19° to the east-northeast, whereas the strata in the hanging wall are strongly deformed and form a fold sequence indicative of top-to-the-south thrusting. Slates experienced extensive bedding plane slippage during crustal shortening (Fig. 9B), forming S1 cleavages and duplexes. In contrast, interbedded thick and more competent metasandstones are characterized by weak deformation with well-preserved S0 surfaces. Within the Tuanshanbei adits, we also identified a thrust fault (Fig. 9F-H) in carbonaceous slates that dips toward the north-northeast at an angle of 70° to 73°. Numerous subsidiary fractures can be observed in the core of the fault over a width of 80 cm. Kinematic indicators, such as asymmetric boudins and shear fractures, indicate a top-to-the-south thrusting. An EW- to WNW-trending crenulation lineation (Fig. 9G) developed in the near-horizontal S0 metasandstone beds in the hanging wall of the fault points to a near north-south shortening.
In general, the numerous mesoscopic structures identified in the EW- to WNW-trending deformation zone consistently indicate a near N-S–directed crustal shortening. The tight NE-trending structures in the Neoproterozoic basement metasediments initially formed during the collision between the Yangtze and Cathaysia blocks and were subsequently refolded during middle Paleozoic intracontinental orogeny (D1) under a N-S–trending compressive stress field (Hunan Bureau of Geology and Mineral Resources, 1988; Li et al., 2013; Zhou et al., 2021). The D1 reworking of these Neoproterozoic tight near NE-trending structures is evidenced by many E-W– to WNW-striking folds and brittle-ductile shear zones, steeply N- or S-dipping cleavages, and fracture zones within the basement metasedimentary rocks (Hunan Bureau of Geology and Mineral Resources, 1988; Zhang, L., et al., 2019; Zhou et al., 2021). The D1 deformation is also compatible with the approximately EW- to WNW-trending folding and thrust faulting observed in pre-Devonian basement rocks in the South China block that have been interpreted to be associated with the middle Paleozoic orogeny (Hunan Bureau of Geology and Mineral Resources, 1988; Hao et al., 2010; Bai et al., 2012; Wang et al., 2013). This is further supported by a hydrothermal rutile U-Pb date (416.1 ± 7.3 Ma; Zhan et al., 2022) for veins hosted within the EW- to WNW-trending brittle-ductile shear zones in the Hengjiangchong deposit and the Ar-Ar dating on biotite and muscovite (ca. 450–430 Ma) for near E-W–trending ductile shear zones exposed in the eastern part of the central Jiangnan orogen (Li et al., 2016).
D2: NW-SE–directed shortening
A second stage of major deformation is characterized by NE-trending brittle-dominated structures, including thrust faults and folds developed in the Neoproterozoic Lengjiaxi Group metasedimentary rocks and Devonian-Triassic sedimentary sequences (Figs. 3, 9). The EW- to WNW-directed structures are displaced or tectonically reworked by the NE-striking faults at both regional and outcrop scales (e.g., 21TS02; Fig. 3). In places, the EW- to WNW-directed structures (dipping to the north at 40°–46°) are overprinted by a NE-striking high-angle deformation zone (dipping to the west-northwest at 78°). In the Tuanshanbei adits, subhorizontal EW- to WNW-striking gold-bearing veins are cut by moderately dipping NE-striking gold-bearing veins (Fig. 9K, L). Imbricate thrust structures are developed in the Lengjiaxi Group slates (Fig. 9C), which dip 18°–37° to the west-northwest, indicating a top-to-the-southeast contraction. In addition, S2 cleavages (Fig. 9D) and interlayer striations (Fig. 9E), which developed in the NW-dipping Lengjiaxi Group strata, also indicate a top-to-the-southeast sense of thrusting (e.g., 21TS10, 21TS11). Conjugate shear fractures striking 295° to 315° and 340° to 353° (Fig. 9I, J), NE- to NNE-trending thrust faults that cut and displace pre-Triassic folds (Fig. 3), and NE- to NNE-trending folds at various scales in the Liuyang-Liling ore field all point to a NW-SE–directed crustal shortening (D2 deformation). The D2 deformation also caused development of NW-directed extensional-shear fractures, which is compatible with the formation of the ore-hosting breccia in some Q3 veins and in NW-striking veins at the nearby ca. 220 Ma Zhengchong deposit (Liu et al., 2019; Sun, 2021). The NE-striking Q3 moderately dipping veins may have resulted from extension after the D2 NE-directed regional shortening.
The D2 deformation has been widely recorded in some of the Late Triassic gold deposits in the Liuyang-Liling ore field (Zhengchong, Yanlinsi, Lishuopo, and Xiaojiaoshan deposits), such as shown by the ore-hosting NW-striking extensional shear zones and fractures and NE-NNE–striking fault zones that are parallel to the axial plane of overturned anticlines (Sun, 2021; Wang et al., 2022; Tan, 2022; Zhan et al., 2022). The D2 NW-SE–directed crustal shortening is also compatible with the NE-SW–trending fold-and-thrust structures developed in the entire Neoproterozoic to Middle Triassic sedimentary rock sequence of the South China block during the Triassic intracontinental orogeny (Shu et al., 2021; Wang et al., 2021).
Middle-Late Jurassic northeast-southwest thrusts and fault-related folds associated with northwestward subduction of the Paleo-Pacific plate and Cretaceous brittle normal faulting resulting from rollback of the plate are extensively documented in the central Jiangnan orogen (Li et al., 2016). Accordingly, dating of the gold mineralization at the Dayan occurrence in the Wangu gold field on the opposite side of the Changsha-Pingjiang fault from Tuanshanbei (Fig. 1) indicates some gold in the region definitely formed as recently as the earliest Cretaceous. Dayan mineralization is constrained by a pre-ore zircon U-Pb age of 142 ± 2 Ma and a post-ore muscovite 40Ar/39Ar age of 130.0 ± 1.4 Ma (Deng et al., 2017). The Cretaceous gold mineralization in the Wangu gold field has been considered as forming during the reactivation of the middle Paleozoic WNW– to E-W–trending fracture zones (Deng et al., 2017; Zhou et al., 2021) during the transition from plate subduction to plate rollback. However, no Cretaceous mineralization ages have yet been reported from the Liuyang-Liling gold field. Although identified NE- to NNE-striking structures in the gold field that are suggested to have formed in the Cretaceous (e.g., Li et al., 2016; Zhou et al., 2021) are spatially associated with the Tuanshanbei gold mineralization, they are clearly postmineralization features (Fig. 6B).
Sampling and Analytical Methods
All the samples for study were collected from the underground workings at the 280-m main mining level. Before mineral separation, all wall-rock material was carefully cut from the ore samples with a SH300 wire-cutting machine, and then samples were rinsed thoroughly. Zircon grains (15 from the Tuanshanbei granodiorite [sample TS-GS-2] and seven from a Q2 vein [sample TS1-2-7]) were selected for U-Pb dating analyses. Because of the small size of zircon grains in the Q3 veins (<5 μm), zircons from the younger veins were not studied. Twenty-eight and 21 monazite grains from Q2 veins (TS1-2-1 and TS1-2-3) and Q3 veins (TS1-3-1 and TS1-3-9), respectively, were selected for geochemical and U-Th-Pb dating analyses. Ten ankerite samples (six from Q2 and four from Q3 veins) were selected for Sm-Nd dating. Ankerite, zircon, and monazite were separated by standard heavy liquid and magnetic separation techniques at the Geological Geomatics Institute of Hebei and then handpicked under a binocular microscope. Zircon and monazite grains were mounted in epoxy and polished.
Mineral scanning electron microscope (SEM)-based back-scattered electron (BSE) imaging and cathodoluminescence (CL) imaging
Samples (Q2: TS1-2-1, TS1-2-3, TS1-2-7; Q3: TS1-3-1, TS1-3-5, and TS1-3-9) were first prepared as polished petrographic thin sections for microscopic observation to recognize the morphology, textures, and paragenesis of ore-related minerals. Samples were examined with a JEOL JCM-7000 SEM at the Collaborative Innovation Center for Exploration of Strategic Mineral Resources, China University of Geosciences (Wuhan), the School of Geosciences and Info-Physics, Central South University, and the Electron Microprobe Laboratory of the Chinese Academy of Geological Sciences, Beijing, respectively. The operation was conducted at an accelerating voltage of 15 kV, and the raw data were corrected using proprietary JEOL software. Detailed procedures were described in Zhang and Yang (2016). Zircon cathodoluminescence (CL) images (Tuanshanbei granodiorite: sample TS-GS-2; Q2 vein: sample TS1-2-7) were performed using a TESCAN MIRA3 field emission-scanning electron microprobe (FE-SEM) at the Testing Center, Tuoyan Analytical Technology Co. Ltd. (Guangzhou, China). After the samples were gold-coated, SEM-CL images were acquired under an acceleration voltage of 10 kV, a beam current of 15 nA, and a magnification of 300 to 500×. Some images are presented as mosaics of multiple CL images because of the small field of view.
Electron probe mapping
The compositional element maps of monazite (Q2: TS1-2-1; Q3: TS1-3-9) were determined using a JEOL JXA-8230 electron microprobe at the Wuhan SampleSolution Analytical Technology Co., Ltd., Wuhan, China. Compositional element mapping of Si and Au for monazite grains from Q2 and Q3 veins was performed using a 15-kV accelerating voltage, a 5-nA beam current, and a beam diameter of 0.4 μm. Detailed procedures were described in Zhang and Yang (2016).
Zircon and monazite U-Th-Pb dating methods
Zircon and monazite U-Th-Pb dating and rare earth element analysis were completed by LA-ICP-MS at the Wuhan SampleSolution Analytical Technology Co., Ltd., China. An Agilent 7700e ICP-MS instrument was used to acquire ion signal intensities. Helium was applied as a carrier gas. Argon was used as the make-up gas and mixed with the carrier gas via a T connector before entering the ICP. A wire signal-smoothing device is included in this laser ablation system, by which smooth signals are produced even at very low laser repetition rates down to 1 Hz. The spot size and frequency of the laser were set to 16 μm and 2 Hz, respectively. Zircon standard 91500 (1062 ± 4 Ma; Wiedenbeck et al., 2004), monazite standard 44069 (424.9 ± 0.4 Ma; Aleinikoff et al., 2006), and glass NIST610 were used as external standards for U-Th-Pb dating and rare earth element calibration, respectively, and the monazite standard Trebilcock (TRE; 272 ± 2 Ma; Tomascak et al., 1996) and the zircon standard Plešovice (PLE; 337.13 ± 0.37 Ma; Sláma et al., 2008) were used as a secondary standard to monitor zircon data quality. The errors for individual analyses are reported at the 1σ level, and the weighted mean 206Pb/238U ages are quoted at the 95% confidence level. The Excel-based software ICPMSDataCal was used to perform offline selection and integration of background and analyzed signals, time-drift correction, and quantitative calibration for rare earth element analysis and U-Pb dating (Liu et al., 2010).
Ankerite Sm-Nd dating
The ankerite samples were analyzed on a Triton Ti thermal ionization mass spectrometer (TIMS) at the Wuhan Center, China Geological Survey. One powdered subsample was digested with 145Nd + 149Sm mixed diluent and then dried for mass spectrometric analysis of Sm and Nd content. The other subsample was digested with hydrochloric acid, after which the acid was removed and the residue was extracted in dilute hydrochloric acid. After the desorption solution without diluent was dried, the residue was extracted with dilute hydrochloric acid, and then the Nd was separated and purified using the P507 organic resin column prior to isotopic analysis. The Nd isotope composition was normalized to 146Nd/144Nd = 0.7219. The chemical analyses of the samples were performed in an ultrafiltration laboratory using soluble polytetrafluoro-ethylene.
All the analytical processes were monitored using the Chinese National standard protocol GBW04419 and GSW04-3258-2015. Their measured average values are Sm/(10–6) = 3.008 ± 0.007, Nd/(10–6) =10.05 ± 0.01, 143Nd/144Nd = 0.512718 ± 6 (2σ) (n = 4), and 143Nd/144Nd = 0.512436 ± 5 (2σ), consistent with the standard protocol values within error range. The background compositions for Nd and Sm in the entire process were 6 × 10–11 g (n = 7) and 5 × 10–11g (n = 7), respectively. The Sm-Nd isochron age calculations were conducted using Isoplot/Ex_ver3 (Ludwig, 2003).
Magmatic zircon U-Pb age
The CL images of zircons from the Tuanshanbei granodiorite distal to the veins revealed idiomorphic shapes, with the development of oscillatory and rhythmic zoning (Fig. 10A). The 15 zircon grains have diameters ranging from 90 to 160 μm, with length/width ratios ranging from 1:1 to 3:1. They are fresh and do not record evidence of hydrothermal alteration. Their Th/U ratios are between 1.3 and 2.1 (App. Table A2), indicating that the zircons have primary magmatic features. Fifteen spots on 15 grains have variable U, Th, and rare earth element (REE) contents ranging from 357 to 2,684, 534 to 5,552, and 746 to 4,011 ppm, with averages at 870, 1,498, and 1,706 ppm, respectively. These zircon grains yielded 206Pb/238U ages from 448.6 ± 5.8 to 425.9 ± 5.5 Ma, forming an intercept age of 433.6 ± 5.4 Ma (1σ, mean square of weighted deviates [MSWD] = 1.4) and weighted mean 206Pb/238U age of 437.2 ± 4.2 Ma (1σ, MSWD = 1.5) (Fig. 10A, B); the 437.2 ± 4.2 Ma is interpreted as the best estimate of crystallization age for the Tuanshanbei granodiorite. It also places an upper limit on the timing of gold mineralization.
Monazite mineralogy and U-Th-Pb geochronology
Monazite grains have a variety of textures, shapes, and sizes (Q2 monazite: 5–400 μm; Q3 monazite: 1–200 μm; Figs. 8, 11). Monazite, observed as euhedral to subhedral crystals or clusters, is present in Q2 and Q3 veins, either disseminated within both quartz and ankerite or coexisting with pyrite, arsenopyrite, gold, and rutile (Fig. 8). Monazite is often attached to grains of pyrite and arsenopyrite or may be replaced by the sulfides along monazite grain margins (Fig. 8). In addition, monazite from Q3 veins also coexists with Bi-, Sb-, Au-, Te-, and Ni-bearing minerals (Fig. 8D, E). Tiny inclusions of gold microparticles, quartz, and ankerite are often enclosed in the monazite (Fig. 11).
The chemical compositions of the Q2 (30 spots from 28 grains) and Q3 monazite (21 spots from 21 grains) are listed in Appendix Table A3 and Figure 12. There are distinct chemical differences that fingerprint the grains collected from the two different vein generations. The monazites in the Q2 and Q3 veins do have a similar REE-normalized pattern with enrichment in light (L)REEs (Fig. 12). However, it is noteworthy that the Eu/Eu* negative anomalies for the Q2 monazites range from 0.45 to 0.89, whereas a narrower range of 0.87 to 0.90 characterizes Q3 monazites. The LREEs, middle (M) REEs, and total REEs for Q2 monazites are lower but heavy (H)REEs are higher than those in the Q3 monazites.
The LA-ICP-MS analysis results of Q2 and Q3 monazite grains are listed in Appendix Table A4 and illustrated in Figure 13A and B. All analytical monazite grains are subhedral to broken fragments, with a length of 50 to 200 μm. All analyses conducted on the monazite grains are from material that is unaltered and free of inclusions and cracks. Thirty analyses on 28 Q2 monazite grains define a Tera-Wasserburg lower intercept age of 415.1 ± 2.1 Ma with MSWD = 1.03 (Fig. 13A). The Th and U contents for Q2 monazite range from 2,887 to 23,346 and 29 to 692 ppm, respectively, with Th/U = 16.0 to 302.7. For Q3 monazite, 21 analyses on 21 spots from 21 different grains give results of Th = 879 to 76,526 ppm, U = 157 to 3,915 ppm, and Th/U = 0.9 to 363.6 and yield the Tera-Wasserburg lower intercept age of 234.3 ± 1.1 Ma (MSWD = 1.2; Fig. 13B).
Hydrothermal zircon mineralogy and U-Pb geochronology
Zircon grains from the auriferous Q2 quartz vein (sample TS1-2-7) are 40 to 80 μm in width and 60 to 150 μm in length. These grains are mostly anhedral to subhedral and coexist with arsenopyrite, pyrite, rutile, quartz, ankerite, and monazite (Fig. 8A). In CL images, these grains display variable internal structure of homogeneous, broad, and oscillatory zoning (Fig. 14A-C). These zircon grains also contain inclusions of quartz as observed in BSE images (Fig. 14C, D). The LA-ICP-MS U-Pb isotope analyses for Q2 zircon were conducted on the zircon grains free of inclusions and cracks. Their LA-ICP-MS analytical data are listed in Appendix Table A5. Seven spots on seven grains from auriferous veins have significantly higher U, Th, and REE contents than the zircons from granodiorite, which range from 554 to 5,001, 483 to 9,812, and 4,698 to 7,815 ppm, with averages at 2,961, 4,915, and 6,271 ppm, respectively, and yield the lower intercept age of 411.2 ± 4.0 Ma with MSWD = 0.85 and Th/U = 0.9 to 2.0 (Fig. 13C).
Ankerite Sm-Nd geochronology
Six and four ankerite samples were taken from six Q2 and four Q3 ore-bearing veins, respectively. The Sm and Nd isotope analyses from Q2 ankerite grains are presented in Appendix Table A6, and the corresponding isochron line is shown on Figure 13D. The results of four Q3 ankerite analyses were absent due to their low Sm and Nd values and inability to yield Sm-Nd isochrons. The measured 147Sm/144Nd ratios range from 0.478 to 0.880, and 143Nd/144Nd ratios range from 0.512438 to 0.513508. They give a well-defined isochron age of 410 ± 15 Ma (initial 143Nd/144Nd = 0.511161 ± 0.000070 and MSWD = 3.7).
Geologic and geochronological constraints on two periods of mineralization
The ages of the gold deposits in northeastern Hunan Province have been determined by a number of geochronological techniques (e.g., ESR on hydrothermal quartz, Rb-Sr dating of fluid inclusions in hydrothermal quartz, 40Ar/39Ar dating on the hydrothermal biotite and muscovite, and U-Pb dating of hydrothermal rutile) in an attempt to relate gold mineralization to the evolution of prolonged orogenic processes and multistage hydrothermal events. Results have yielded two wide ranges of reported mineralization ages that appear to bracket the main ore-forming events very broadly at times between 462.0 ± 18.0 and 416.1 ± 7.3 Ma and 222.4 ± 9.4 and 130.0 ± 1.4 Ma (e.g., Fig. 15; Han et al., 2010; Huang et al., 2012; Deng et al., 2017; Sun, 2021; Tan, 2022; Wang et al., 2022; Zhan et al., 2022). The two broad ranges reflect the facts that the ESR (Yanlinsi deposit; Huang et al., 2012), Rb-Sr (Tuanshanbei; Han et al., 2010), and Ar-Ar dating are susceptible to problematic issues such as very low Rb and Sr contents, multiple generations of quartz, abundance of secondary fluid inclusions, less cumulative 39Ar released (i.e., 48.9% in Lishupo biotite; Wang et al., 2022), and isotopic resetting during overprinting thermal events and deformation (i.e., Zhengchong muscovite; Sun, 2021). Furthermore, a reported LA-ICP-MS in situ rutile U-Pb age from NE-striking gold-bearing veins in the Xiaojiashan deposit (Tan, 2022) warrants caution because of the chosen small beam spots (~10 μm). Recently, with clear geologic relationships and supporting petrographic evidence, a reliable LA-ICP-MS U-Pb age of hydrothermal rutile in ores at the Hengjiangchong deposit (416.1 ± 7.3 Ma; Zhan et al., 2022), just a few kilometers north of Tuanshanbei, indicated at least some of the gold in this region was certainly deposited in the mid-Paleozoic. Although previous studies suggest there are likely both mid-Paleozoic and early Mesozoic periods of gold mineralization in the Liuyang-Liling gold field, the ambiguous paragenetic associations and above-stated inherent problems of dated materials have led to a poor understanding of the timing of events within different structural generations.
We identified two generations of gold-bearing veins at the Tuanshanbei deposit, which are also observed throughout much of the Liuyang-Liling ore field. The first-generation veins are subhorizontal to shallowly dipping and east-west to west-northwest striking, whereas the second-generation veins are moderately to steeply dipping with a northwest trend and moderately dipping with a northeast/north-northeast trend (App. Table A1). Our field observations and new geochronological data indicate that the two generations of gold-bearing veins at the Tuanshanbei deposit are broadly constrained in age by the Silurian granodiorite host rock (i.e., 437.2 ± 4.2 Ma) and the crosscutting Late Triassic diabase dikes (225.2 ±1.9 Ma; Zhou et al., 2022).
Origins of ankerite, monazite, and zircon
To fully interpret the ore-forming ages, key questions must be addressed so as to convincingly argue the dated minerals record gold events (Schandl and Gorton, 2004; Deng et al., 2020). Ankerite is texturally associated with quartz and gold-bearing pyrite and arsenopyrite (Figs. 6–8), and the consistent spatial and paragenetic relationship indicates that ankerite is essentially synchronous with gold deposition. The CO2-bearing nature of the gold-forming fluids (Tan, 2021) also supports a connection between gold and formation of an abundance of hydrothermal carbonate. Furthermore, three potential origins for monazite and zircon hosted in the ore-bearing veins are possible: (1) detrital monazite and zircon grains are incorporated in the veins from the Neoproterozoic metasedimentary strata; (2) these are igneous grains inherited from the hosting intrusion; or (3) these are syngold grains precipitated from two hydrothermal events.
As introduced above, Th contents in Q2 and Q3 monazites (App. Table A3) are lower than those in typical magmatic monazite (ThO2 > 10 wt %; Montel et al., 2018). In fact, the very low Th contents are criteria for defining hydrothermal monazite due to the lower solubility of Th4+ relative to LREE3+ in crystallization of the hydrothermal monazite (Schandl and Gorton, 2004). Our monazite grains have a REE-normalized pattern with insignificant Eu negative anomalies, thus contradictory to what would be expected for magmatic monazite (Fig. 12; e.g., Smith et al., 2000; Schandl and Gorton, 2004; Williams et al., 2007). The monazite U-Pb ages for the Q2 and Q3 auriferous veins (415.1 ± 2.1 and 234.3 ± 1.1 Ma) are clearly younger than those for the ore-hosting Tuanshanbei granodiorite (437 Ma) and Lengjiaxi Group metasedimentary rocks (ca. 822 Ma; Gao et al., 2011). Furthermore, microscopic observations reveal that monazite precipitation was synchronous with native gold, arsenopyrite, pyrite, quartz, and ankerite crystallization (Fig. 8), indicating that the analyzed monazite grains from the ore veins are hydrothermal in origin.
Inherited core and old apparent ages are absent in all analyzed zircon grains from the Q2 veins, arguing against the above-mentioned inherited Neoproterozoic detrital country-rock source. The grains have very high Th and U contents (Th = 483–9,812 ppm, avg = 4,915 ppm; U = 554–5,001 ppm, avg = 2,961 ppm; App. Table A5), which are much higher than those zircons for igneous rocks (Hoskin, 2005) and specifically the Tuanshanbei granodiorite (Th = 534–5,552 ppm, avg =1,498 ppm; U = 357–2,684 ppm, avg = 870 ppm; App. Table A2). The grains also have very high REE contents (REE = 4,698–7,815 ppm, avg = 6,271 ppm; App. Table A5), which are much higher than those zircons for the Tuanshanbei granodiorite (REE = 746–4,011 ppm, avg = 1,706 ppm; App. Table A2). In addition, the U-Pb age of 411 Ma for the Q2 vein zircons is significantly lower than the 437 Ma zircon age for the Tuanshanbei granodiorite. Such signatures, along with the absence of older inherited zircons, rule out the possibility of a magmatic origin. In addition, evidence that these dated zircons are syngold grains precipitated from the hydrothermal ore fluids is further supported by (1) the coexistence of zircon, rutile, monazite, arsenopyrite, and pyrite in the quartz-ankerite veins (Figs. 8A, 14C, D) and (2) preservation of the abundant fluid inclusions in grains (Fig. 14E). In the dated ore samples, ankerite grains are closely intergrown with sulfides and native gold (Figs. 6–8). The intergrown mineral phases crystallized almost simultaneously from the same hydrothermal fluid. Six ankerite samples from Q2 ore-bearing veins give a well-defined isochron age of 410 ± 15 Ma.
The synchronous age measurements for hydrothermal monazite, zircon, and ankerite, all postdating local late Silurian magmatism in the gold field, are consistent with an Early Devonian gold event in the gold field. Seven hydrothermal zircons and 28 hydrothermal monazite grains from the Q2 veins yield U-Pb ages of 411.2 ± 4.0 and 415.1 ± 2.1 Ma, respectively. Six ankerite samples from the Q2 veins yield an Sm-Nd isochron age of 410 ± 15 Ma. Three ages from three different ore-related minerals are similar within error, reflecting formation of the Q2 veins at 415 to 410 Ma. Such ages are consistent with the existing rutile U-Pb mineralization age of 416.1 ± 7.3 Ma from elsewhere in the Liuyang-Liling ore field (e.g., Hengjiangchong deposit; Zhan et al., 2022). In addition, 21 hydrothermal monazite grains from the Q3 veins define a U-Pb age at 234.3 ± 1.1 Ma. This is 10 to 15 m.y. older than other estimates for a second period of gold deposition elsewhere in the Liuyang-Liling ore field, as had been implied based on above-mentioned unreliable isotopic methods (i.e., Han et al., 2010; Huang et al., 2012; Sun, 2021; Tan, 2022; Wang et al., 2022), but nevertheless confirms a Late Triassic gold event. The estimated U-Th-Pb closure temperatures of monazite and zircon are higher than 750°C and 800° to 900ºC, respectively (Cherniak and Watson, 2001; Cherniak et al., 2004; Cherniak, 2010). Thus, hydrothermal zircon and monazite grains would start their U-Th-Pb clock and remain closed as soon as they have formed, which further indicates that these dates for hydrothermal zircon and monazite reliably represent the age of gold mineralization rather than resetting ages. Thus, our new geochronological results have identified two discrete gold mineralization events at 415 to 410 and ca. 234 Ma that together were responsible for gold concentration at the Tuanshanbei deposit and collectively within the Liuyang-Liling ore field. This also implies that the individual gold mineralization events lasted for a relatively short time, rather than over a much broader hydrothermal history inferred from previously published results (i.e., 462.0 ± 18.0–416.1 ± 7.3 and 222.4 ± 9.4–130.0 ± 1.4 Ma; Han et al., 2010; Huang et al., 2012; Deng et al., 2017; Sun, 2021; Tan, 2022; Wang et al., 2022; Zhan et al., 2022).
Gold genesis model
The geology and geochemical data from gold-bearing veins at Tuanshanbei (e.g., δD, δ18O, δ13C, and fluid inclusions: Tan, 2021), as well as the lack of spatially temporally over-lapping magmatism, are consistent with an orogenic gold deposit model (Groves et al., 1998) emphasizing a genesis from metamorphic devolatilization processes (e.g., Phillips et al., 1996; Goldfarb and Groves, 2015). In such a model, fluid and metals are released across the greenschist-amphibolite facies boundary, where chlorite and pyrite are broken down and a substantial volume of gold-bearing fluid is focused into major structures (McCuaig and Kerrich, 1998; Pitcairn et al., 2006, 2021; Phillips and Powell, 2010; Tomkins, 2010; Zhong et al., 2015). Although the exposed basement ore-hosting rocks in Tuanshanbei are deformed Neoproterozoic subgreenschist to lower greenschist facies metasandstone, siltstone, and slates (Hunan Bureau of Geology and Mineral Resources, 1988; Wang et al., 2005; Zhang et al., 2018), it is well recognized that middle and lower crustal high-grade metamorphism and D1 ductile deformation characterized much of the region underlain by the Lengjiaxi Group metasediments during early to middle Paleozoic orogeny (Wang et al., 2007, 2013, 2021). In addition, the Neoproterozoic Cangxiyan Group strata, which are directly beneath Lengjiaxi Group strata, comprise a series of greenschist to amphibolite facies metamorphic rocks (Tang et al., 1999; Wu et al., 2005; Gao et al., 2011). Zircon grains yielded a 206Pb/238U age of 428.4 ± 5.6 Ma for metatuff from the Cangxiyan Group (Gao et al., 2011), which might be indicative of a middle Paleozoic metamorphic event. The large crustal melt bodies emplaced into the southern edge of the Liuyang-Liling gold field at ca. 434 Ma (Fig. 1B), as well as the similar age for the smaller intrusive body hosting the Tuanshanbei deposit, indicate the early stages of a large thermal episode capable of generating and mobilizing a large metamorphic fluid volume sometime during the subsequent 20 m.y. (Fig. 16).
Post-Devonian, the South China block has been situated in a stable coastal-neritic depositional environment of an archipelagic framework (Shu et al., 2021), which is marked by the continuous sedimentation from the Late Silurian to the Early Triassic in northeastern Hunan Province (Figs. 2, 3). During the Early-Middle Triassic, the South China block was deformed by collisions with the rigid North China and Indochina blocks to the north and south, respectively (e.g., Wang et al., 2013, 2021; Zhang, G.W., 2013), resulting in the generation of the ~250 to 225 Ma structural elements and ca. 240 to 234 Ma amphibolite facies metamorphism in the southern South China block (Wang, Y.J., et al., 2007, 2013; Xiang et al., 2008a, b; Wang, Z.L., et al., 2022). The δ34S values of gold-bearing pyrite and arsenopyrite in Late Triassic veins at the Zhengchong, Yanlinsi, and Xiaojiashan deposits range from –2.1 to –0.1‰, –4.3 to –1.1‰, and –3.4 to –1.0‰, respectively, and are interpreted by past workers as typical of magmatic sulfur (Liu et al., 2019; Sun, 2021; Tan, 2022; Zhang et al., 2022). However, the structural style of the Q3 veins, and a lack of typical magmatic brittle stockworks or sheeted veins in association with some type of plutonic body, argue against a magmatic-hydrothermal orogenic gold model. Furthermore, the Q3 monazite grains have a REE-normalized pattern with insignificant Eu negative anomalies, thus inconsistent with magmatic-hydrothermal systems that would be expected to exhibit large negative Eu anomalies reflecting earlier plagioclase fractionation in the granitic melt (Schaltegger et al., 2005; Taylor et al., 2015). In addition, the low-salinity, CO2-rich nature of the fluid inclusions from the Q3 veins (Tan, 2021) is also similar to recognized aqueous-carbonic ore-forming fluids produced by greenschist to amphibolite facies metamorphism (Ridley and Mengler, 2000; Goldfarb et al., 2005) rather than fluids typical of magmatic water (Fig. 16). The Cangxiyan Group tremolite rocks contain a series of zircon grains with 206Pb/238U ages of 242 ± 4.6 to 237 ± 13 Ma (Tan et al., 2022), indicating they were locally further metamorphosed in the Late Triassic roughly at a time consistent with the Q3 monazite U-Pb age. Together with existing geologic and geochemical evidence (Fig. 16), we infer that the Late Triassic gold mineralization is consistent with a model invoking generation of an ore-forming fluid volume from the Cangxiyan Group metamorphic rocks.
Conclusions of this study are as follows:
The Tuanshanbei gold deposit includes two generations of auriferous quartz-ankerite-pyrite-arsenopyrite veins (Q2 and Q3). The Q2 veins were formed throughout the near S-N–directed shortening (D1) within subhorizontal and low-angle-dipping EW- to WNW-striking fault zones. In contrast, the overprinting and higher-grade Q3 veins contain more abundant ankerite and base metal sulfides and are both moderately to steeply dipping with a northwest trend and moderately dipping with a northeast to north-northeast trend, which accompanied NW-SE–directed shortening (D2).
Textural and geochemical characteristics indicate that the monazite, zircon, and ankerite grains paragenetically associated with native gold and gold-bearing pyrite and arsenopyrite in the two auriferous vein sets on the Tuanshanbei deposit are hydrothermal in origin. These findings support the conclusion that geochronological measurements of the gangue phases can yield reliable ages of gold deposition.
In situ LA-ICP-MS U-Pb dating of the monazite and zircon and Sm-Nd dating of the ankerite from Q2 veins in the Tuanshanbei deposit yield ages of ca. 415 to 410 Ma. Dating of monazite from the Q3 veins gives an age of 234 Ma. These data suggest the Tuanshanbei gold resource was formed by two distinct hydrothermal episodes a few hundreds of millions of years apart. These ages are consistent with those of other orogenic gold deposits in northeastern Hunan Province that are most commonly recognized to have formed in either the mid-Paleozoic or the Late Triassic.
The Early Devonian and Late Triassic gold ores are best interpreted as forming from metamorphic devolatilization processes within the crust and subsequent structural focusing of resulting fluids. Moderate- to high-temperature orogenic events are recognized during both epochs, and metamorphic minerals associated with both times are recognized in the basement rocks. It is most likely that different parts of the Neoproterozoic basement sequence were heated through the greenschist-amphibolite boundary at different times, with generated fluids in both cases eventually migrating into the same major and then more local structural conduits.
There are numerous examples in the recent literature where large orogenic gold deposits are characterized by multiple dates of ore formation spread across time intervals of hundreds of millions of years. In many cases, some suggested ages of hydrothermal activity are not even associated with recognized deformational or thermal events, and the significance of the multiple measured dates is questionable. However, our findings at Tuanshanbei provide a definitive example of an orogenic gold resource formed during two distinct tectonothermal events, both of which are widely recognized in the region and are temporally distinct. This demonstrates how major fluid conduits for orogenic gold formation may be active at multiple times over a lengthy geologic period. If crustal fluids are released via devolatilization reactions at different times and at different locations within fertile country rocks, then decreasing pressure gradients in the direction of major structures can consistently lead to fluid focusing toward such conduits.
We appreciate the help of Yuejun Wang and Yang Wang from Sun Yat-sen University and Changsheng Lan from the Jinsha Mining Co., Ltd., for the field investigation, and Zhenyu Chen and Lu Zhang for the analysis. Editor-in-Chief Larry Meinert, the associate editor, Ian Honsberger, and two anonymous reviewers are acknowledged for their constructive reviews and suggested revisions, which substantially improved this work. This research was financially supported by the National Key Research and Development Program of China (2022YFC2903603), the National Natural Science Foundation of China (42172085, 42272100), the project (2021RC4055) funded by the Innovation Team of Hunan Province, and Hunan Natural Science Foundation (2021JJ30809).
Cheng Wang is currently a postdoctoral research associate at the Sun Yat-Sen University, China, working on the ore genesis and structural controls of orogenic gold deposits and granite pegmatite-related Li-Rb-Cs mineralization in Hunan Province, South China. He received his B.Sc. (2013) degree from the Hefei University of Technology, and his M.Sc. (2016) and Ph.D. degrees (2019) in economic geology from the Central South University.